Electrochemical fuel cells can be used in a vast array of applications as a power source, including as an alternate power source to the internal combustion engine for vehicular applications. An electrochemical fuel cell contains an anode and a cathode, with an interstitial space maintained between the two electrodes, where the fuel that is oxidized at the anode passes through as ions to be reacted with oxygen which is reduced at the cathode. One preferred embodiment of the electrochemical fuel cells is a proton exchange membrane (PEM), where hydrogen (H2) is used as a fuel source or reducing agent at the anode. In a PEM fuel cell, oxygen (O2) is typically provided as the oxidizing agent at the cathode, either in pure gaseous form or combined with nitrogen and other inert diluents present in air. During operation of the fuel cell, electricity is garnered by electrically conductive elements proximate to the electrodes via the electrical potential generated during the reduction-oxidation reaction occurring within the fuel cell.
One of the difficulties in implementing the fuel cell as a power source for vehicular applications has been supply issues for delivery of hydrogen to the vehicle, as an infrastructure for hydrogen distribution is not currently available in most areas. One approach to overcoming these difficulties has been the advent of a vehicular fuel processor which is a permanent onboard fixture. The fuel processor is capable of converting a hydrocarbon fuel into a hydrogen feed stream for the fuel cell. Preferred hydrocarbon fuels available in current distribution systems include low molecular weight alcohols (e.g. methanol or ethanol) or other hydrocarbon fuels (e.g. gasoline).
One preferred method of processing a hydrocarbon fuel has been through a multiple reactor fuel processing unit. A typical preferred sequence of reactions in the fuel processor following the introduction of the hydrocarbon fuel can include: a primary reactor and one or more CO cleanup reactors, where the hydrogen-containing gas created through the fuel processor reactions is introduced to the fuel cell anode. One preferred embodiment of the fuel processor is where the primary reactor is an autothermal reformer (ATR) which combines the partial oxidation reaction (POx) and the steam reforming (SR) reaction within one reactor, the primary CO clean-up reactor is a water gas shift reactor (WGS) and the secondary CO clean-up reactor is a preferential oxidation reactor (PrOx). All of the above mentioned reactions are preferably facilitated by catalysts, which enable lower temperature operating ranges to achieve reaction energy activation levels and higher reaction conversions.
Start-up conditions within the fuel processor pose challenges in the implementation of fuel cell technology. As used herein, “start-up” conditions generally refers to transient operating conditions when the fuel processor is transitioning or being engaged from a cold state to normal, or steady-state, ranges for operating temperature, fuel delivery and hydrogen output. “Normal”, “steady-state”, “non-start-up” or “run mode” conditions refer to the operating conditions when temperatures are within typical operating ranges and hydrogen-containing effluent is reformed in the fuel processing unit without detrimental byproduct formation such as unconverted fuel. These terms may further include transient operating conditions that may be the result of varying load requirements on the system, but not relating to start-up conditions. One of the primary issues is that to reach steady-state operations, temperatures in the primary reactor must be stabilized within the range of about 400° C. to about 700° C. Within these temperature ranges, the catalyst within the primary reactor is capable of properly converting hydrocarbon fuel to hydrogen rich effluent.
When temperatures in the primary reactor are below this range, reaction conditions can lead to extremely detrimental results in the catalysts both within the fuel processor and the fuel cell stack. Condensation can occur on the catalysts within the fuel processor on cold start-up, or carbonaceous deposits may form on the catalytic surfaces. Incomplete reactions and low reaction conversion efficiency in the fuel processor can result in high concentrations of carbon monoxide or carbon (soot) deposition that poison the downstream reactors and the fuel cell stack, as well as lowering power output from the fuel cell. Transitions during cold start-up pose numerous problems, and to reach the proper stable temperature ranges without detrimental side effects, either the fuel processor must be allowed to run for a relatively long duration of time off-line (i.e., bypass the fuel cell stack) to reach steady-state operations, or other methods of preheating the fuel processor reactors must be used.
A fuel cell system typically has at least one combustor, which includes an indirect combustor that receives stack anode exhaust, and further may be capable of transferring heat via a heat exchanger to the fuel processor reactors. Additionally, a fuel cell may have a direct fire combustor within the fuel processor reactors. These combustors are generally employed in heating a system during start-up to achieve steady-state operations from a cold start, as well as potentially providing heat to reactors that carry out endothermic reactions and supplementing energy on high power demand situations.
One preferred combustor is a direct fire inline preheater that is placed upstream of the primary reactor. With this combustor, the fuel in the preheater combustor flame is oxidized to release heat, which is used to raise the temperature of the downstream reactor. The preheater combustor flame can be operated as a fuel lean or fuel rich flame. A fuel lean flame refers to circumstances when the fuel is provided sub-stoichiometrically to the oxygen for a combustion reaction. Conversely, a fuel rich flame refers to the fuel being fed in excess of the stoichiometric level of oxygen. Both fuel lean or rich flames have advantages during start up. However, the fuel rich flame in the preheater combustor is preferred in this start-up scenario, where the equivalence ratio (actual fuel to oxygen delivered divided by stoichiometric fuel to oxygen) is greater than one, and more preferably is greater than 2. Fuel rich operation is preferred to avoid oxidizing conditions on the reactors (especially CuZn water gas shift catalysts) and to permit heating of downstream reactors by staged air addition.
The preferred equivalence ratio varies based on combustor design factors, such as whether the fuel is mixed within the preheater combustion zone. The flame temperature in comparison to the equivalence ratio likewise varies based upon system design parameters. As the fuel to oxygen ratio approaches the combustion stoichiometry the flame temperature increases. The flame temperature reaches a peak, or maximum value at an equivalence ratio of 1. As additional fuel in stoichiometric excess is introduced into the combustion zone, the equivalence ratio increases, and the flame temperature follows a reducing trend. Thus, operating within a fuel rich combustor start-up scenario (with an equivalence ratio greater than 1) provides a flame temperature that is relatively low, avoids oxidizing conditions, and provides fuel for downstream reactions by air staging. The start-up combustor flame temperature operating range is limited at the upper end (i.e., maximum temperature) by the physical properties of the catalyst downstream, which has the potential to degrade above about 700° C. to 1000° C., and at the lower end (i.e., minimum temperature) by the formation of undesirable byproducts including carbon containing compounds which are created when the combustion reaction is incomplete. The combustion by-products may be physically deposited on the catalyst surface, which may block fluid flow and active sites. Further, certain combustion by-products may bind to the active sites, thereby poisoning the catalyst and decreasing activity downstream. This catalyst abuse can also result in a shortened catalyst lifespan.
The start-up combustor flame temperature operating range can be reduced either by increasing the air inlet temperature or by employing a non-premixed combustor zone. However, it is desirable to broaden the operating range of the start-up combustor flame temperature for heating the downstream reactors. A broadened operating range at the minimum, or lower end, reduces the requisite flame temperature and potential catalyst damage due to high temperatures. Thus, there is a need for a rapid start-up system in a fuel processor that provides the ability to operate the combustor flame at lower temperatures during fuel rich operations while providing protection and a longer lifespan for the downstream fuel processor reactors.